Photosensitive controls are utilized in a number of environments where it is desirable to turn a light source on or off depending upon the amount of ambient light. For example, in landscape lighting applications, it may be desirable to automatically turn lights on at dusk and turn lights off at dawn, or alternatively, after a fixed number of hours after dusk. In addition, it may be desirable in some motion sensing or security applications to sense the amount of ambient light to prevent a motion-sensitive light from turning on during the day. One challenge that is encountered with respect to photosensitive controls, however, results from the feedback of light from a controlled light source to the light detector used in determining the ambient light level. In some photosensitive controls, for example, a light detector output is compared to a static threshold that the light source is turned on when the ambient light falls below that threshold, and turned off when the ambient light rises above that threshold. However, when a light source is turned on, a portion of the generated light may be detected by the light detector, and may cause the detector input to rise above the static threshold, and cause the photosensitive control to turn the light back off. In some instances, the light source may flicker or repeatedly cycle on and off as a result of the feedback of light from an activated light source.
Some attempts to minimize the effect of feedback have included shielding a light detector or otherwise placing the light detector in a location that minimizes the amount of light from the controlled light source that is fed back to the detector. However, depending upon where the light source and light detector are installed, surrounding structures such as walls and other reflective surfaces may nonetheless reflect light from the light source back to a light detector. As a result, the amount of light feed back to a light detector may vary from installation to installation, and is thus difficult to eliminate through shielding or placement of the light detector.
Additional attempts to minimize the effects of feedback include using hysteresis to set different on and off thresholds, thus requiring a greater amount of ambient light to be detected to turn a light source off than that used to turn the light source on. It has been found, however, that increasing the “window” between on and off thresholds can inhibit accurate dawn detection, particularly on overcast days.
Other attempts to minimize the effects of feedback include dynamically setting thresholds based on the amount of ambient light sensed by a light detector. One conventional implementation, for example, monitors the infrared output of a fluorescent light and sets an off threshold based upon the amount of infrared light sensed after the fluorescent light is turned on, typically after waiting until the rate of change of the infrared output has decreased and the output has stabilized. Also, in this implementation, a rate of change of the light detector output may be used along with the absolute output to minimize the effects of rapid changes in the light detector output.
One problem associated with the aforementioned implementation, however, is that sensing the rate of change of a light detector output typically requires relatively complex processing. Moreover, sensing the rate of change may limit the overall responsiveness of the light detection circuit.
Therefore, what is needed is a simple and responsive photosensitive control that reduces the adverse effects of feedback from a controlled light source.